1. Field of the Invention
The present invention relates to bandgap reference voltage generator circuits, and in particular, to bandgap-based reference voltage generator circuits with compensation for reducing the temperature coefficient.
2. Description of the Related Art
Electronic systems which require a precision reference voltage typically use a bandgap voltage reference circuit, which is advantageously capable of operating with a low power supply potential. As is well known, the basic principle of a bandgap voltage reference circuit is based upon the summation of the negative temperature drift of the base-emitter voltage (Vbe) of a bipolar junction transistor with an appropriate magnitude of a positive temperature drift of a thermal voltage (Vt) in order to achieve a net zero temperature drift sum.
Referring to FIG. 1, a conventional bandgap reference circuit includes two bipolar junction transistors Q2, Q3 biased by a voltage divider circuit composed of resistors R1, R2, R3 and a diode-connected transistor Q1 and a current sinking circuit IS. The size of the emitter area of transistor Q2 is ten times the size of the emitter area of transistor Q3. The collector currents of these transistors Q2, Q3 are amplified differentially by a differential transconductance amplifier which produces the bandgap reference voltage Vbg (Vref), which, in turn, drives the voltage divider circuit. The diode-connected transistor Q1 introduces a voltage into the voltage divider circuit which has a negative temperature coefficient. The difference between the base-emitter voltages Vbe of transistors Q2, Q3 (.DELTA.Vbe=Vbe(Q3)-Vbe(Q2)) has a positive temperature coefficient. The value of the resulting bandgap voltage Vbg (Vref) can be determined in accordance with Equation 1: ##EQU1##
Equation 1 can be rearranged and written as Equation 2: ##EQU2##
To establish a zero temperature coefficient (OTC) at the expected operating temperature (T.sub.0) Equation 2 is differentiated and set equal to zero. This produces Equations 3 and 4: ##EQU3##
Substituting Equation 4 into Equation 2 produces Equation 5 which defines the reference voltage Vref: ##EQU4##
Referring to FIG. 2, the reference voltage Vref with respect to temperature T is graphed in accordance with Equation 5. From this graph it can be seen that, assuming a bandgap energy voltage V.sub.G0 =1.12 V, a constant m=5, an emitter-base junction constant of R=0.2818, an emitter area ratio A=10 and an operating temperature T.sub.0 =20.degree. C., the reference voltage Vref has a temperature coefficient of approximate 12.6 ppm/.degree. C.
However, as the precision requirements for the operating characteristics of modem electronic systems increase, particular as the magnitude of the available power supply voltage decreases, temperature coefficients of such magnitude become increasingly unacceptable. Accordingly, it would be desirable to have a bandgap-based reference voltage generator circuit with compensation which provides for significantly reduced temperature coefficients. Additionally, it would be further desirable to be able to adjust such compensation and provide for such compensation using standard semiconductor processing techniques.